A comprehensive survey on enhancements and limitations of the RPL protocol: A focus on the objective function

The RPL routing protocol for low power and lossy networks uses the objective function (OF) to build a Destination Oriented Directed Acyclic Graph (DODAG) based on a set of metrics and constraints. The OF has as the main function to select and specify the best parent or the optimal path to reach the destination. However, proposing an adequate objective function in Low Power and Lossy Networks (LLNs) presents a substantial challenge. In this paper, we propose a survey on existing objective functions in LLNs based on a set of metrics. These metrics can define a node or/and link characteristics. We highlight the advantages and the shortcoming of each studied solution. Furthermore, we propose a classification of the used metrics and the criteria of choice. Then, we present a comparative study of the existing OFs in terms of the required performances of the RPL protocol and we provide a deep statistical analysis of all reviewed papers. Finally, we conclude our contribution by highlighting the different issues and challenges that can be exploited for future works. We believe that this survey will help LLNs researchers’ community to easily understand the objective function concept and contributes to improving RPL in this context for further relevant research works.